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Let A=[1sin⁡θ1−sin⁡θ1sin⁡θ−1−sin⁡θ1]A = \begin{bmatrix} 1 & \sin \theta & 1 \\ -\sin \theta & 1 & \sin \theta \\ -1 & -\sin \theta & 1 \end{bmatrix}, where 0≤θ≤2π0 \leq \theta \leq 2\pi. Then which of the following are true?

(A) ∣A∣=2+2sin⁡2θ|A| = 2 + 2 \sin^{2} \theta

(B) ∣A∣=2+sin⁡2θ|A| = 2 + \sin^{2} \theta

(C) minimum value of ∣A∣|A| is 11

(D) maximum value of ∣A∣|A| is 44

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 1

We need to find the determinant ∣A∣|A| of the given 3×33 \times 3 matrix and then determine its minimum and maximum values.


For the matrix A=[1sin⁡θ1−sin⁡θ1sin⁡θ−1−sin⁡θ1]A = \begin{bmatrix} 1 & \sin \theta & 1 \\ -\sin \theta & 1 & \sin \theta \\ -1 & -\sin \theta & 1 \end{bmatrix}, we'll expand along Row 1:

∣A∣=1⋅∣1sin⁡θ−sin⁡θ1∣−sin⁡θ⋅∣−sin⁡θsin⁡θ−11∣+1⋅∣−sin⁡θ1−1−sin⁡θ∣|A| = 1 \cdot \begin{vmatrix} 1 & \sin \theta \\ -\sin \theta & 1 \end{vmatrix} - \sin \theta \cdot \begin{vmatrix} -\sin \theta & \sin \theta \\ -1 & 1 \end{vmatrix} + 1 \cdot \begin{vmatrix} -\sin \theta & 1 \\ -1 & -\sin \theta \end{vmatrix}


Calculating each 2×22 \times 2 determinant:

∣1sin⁡θ−sin⁡θ1∣=1+sin⁡2θ\begin{vmatrix} 1 & \sin \theta \\ -\sin \theta & 1 \end{vmatrix} = 1 + \sin^2 \theta

∣−sin⁡θsin⁡θ−11∣=−sin⁡θ+sin⁡θ=0\begin{vmatrix} -\sin \theta & \sin \theta \\ -1 & 1 \end{vmatrix} = -\sin \theta + \sin \theta = 0

∣−sin⁡θ1−1−sin⁡θ∣=sin⁡2θ+1\begin{vmatrix} -\sin \theta & 1 \\ -1 & -\sin \theta \end{vmatrix} = \sin^2 \theta + 1


Substituting back:

∣A∣=1(1+sin⁡2θ)−sin⁡θ(0)+1(sin⁡2θ+1)|A| = 1(1 + \sin^2 \theta) - \sin \theta(0) + 1(\sin^2 \theta + 1)

∣A∣=1+sin⁡2θ+sin⁡2θ+1|A| = 1 + \sin^2 \theta + \sin^2 \theta + 1

∣A∣=2+2sin⁡2θ|A| = 2 + 2\sin^2 \theta

This confirms option (A) is correct.


Since 0≤sin⁡2θ≤10 \leq \sin^2 \theta \leq 1:

Minimum value occurs when sin⁡2θ=0\sin^2 \theta = 0:

∣A∣min=2+2(0)=2|A|_{\text{min}} = 2 + 2(0) = 2

Maximum value occurs when sin⁡2θ=1\sin^2 \theta = 1:

∣A∣max=2+2(1)=4|A|_{\text{max}} = 2 + 2(1) = 4

This confirms option (D) is correct.


Options (A) and (D) are correct.

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